We have shown that the rate of absorption across the intestine is determined by both the lipid cell membrane and the unstirred water layer (UWL) adjacent to that membrane; thus, the resistance encountered by a molecule during absorption comes from both the lipid membrane and the UWL. In specific situations either of these may become the major resistance and, therefore, the rate limiting step in absorption. In the proposed studies these findings are related to major problems of fatty acid and cholesterol absorption. First, a technique is outlined which allows precise quantitation of unidirectional uptake rates for lipids across the intestinal brush border. Second, experiments are outlined in which it will be determined under what conditions the cell membrane or the UWL are rate limiting to absorption; preliminary experiments have shown that for long-chain fatty acid monomers, steroid monomers and micelles it is the UWL while for bile acid monomers, short-chain fatty acids and other water soluble compounds it is the cell membrane. Third, studies are outlined to determine whether the micelle crosses the lipid cell membrane intact. Fourth, since preliminary experiments indicate that the micelle is not absorbed, other studies are outlined that allow elucidation of the actual mechanisms of transfer of fatty acids and steroids from the micelles into the cell; these studies will test the thesis that all fat absorption takes place by transfer of lipid monomers from the micelle across the aqueous phase into the cell. Finally, studies also will be undertaken that allow a comparison of the thickness of the UWL and the permeability characteristics of the cell membrane in different animal species. These investigations should provide the first detailed description of how lipid moves from the micellar phase of the intestinal contents into the absorptive cell. In addition, these investigations may provide an explanation for some of the important differences that exist in fat and sterol absorption among different animal species.